FIG. 4 is a block diagram of an electrical arrangement of a Low Noise Block downconverter (hereinafter referred to as LNB) 1 which is a typical high impedance circuit of prior art. An LNB, a component which is mounted at the end of antennas such as a BS (Broadcasting Satellite) antenna and a CS (Communications Satellite) antenna, receives and amplifies signals of the order of 12 GHz band transmitted from a satellite, further converts the amplified signals into intermediate frequency signals (hereinafter referred to as IF signal) of the order of 1 GHz band, and outputs the IF signals to a receiver not shown. The LNB 1, which is used for both BS and CS, is provided with respective receiving horns 2 and 3. The LNB 1 selects a target satellite for receiving signals in response to a control pulse sent from the receiver not shown. For this, for example, the control pulse of a maximum amplitude (Vs)=1 VP-P and a frequency (f)=22 kHz is supplied to a terminal 4 via a cable not shown.
That is, the LNB 1 is a high impedance circuit which is provided in a system for receiving satellite broadcasts.
The LNB 1 generally includes a high impedance circuit 5, a low impedance circuit 6 to which a direct current for power source is supplied from the terminal 4, and an LC parallel resonance circuit 7 which is provided between the terminal 4 and the low impedance circuit 6. The high impedance circuit 5 outputs the IF signal to the terminal 4 and is supplied the control pulse from the terminal 4. The low impedance circuit 6 includes a regulator IC 8 which carries out the supply of electricity to the high impedance circuit 5. Also, at the input side of the low impedance circuit 6 provided is an input pass capacitor c1 so as to prevent the oscillation of the regulator IC 8. This causes the low impedance circuit 6 to have a low impedance.
For this, the LC parallel resonance circuit 7 is provided so as to electrically separate the regulator IC 8 having a low impedance from the control pulse. The LC parallel resonance circuit 7 includes an inductor 1 and a capacitor c2 which are connected in parallel, and the value of its resonance frequency agrees with the control pulse's frequency of 22 kHz. A constant of LC is determined by the equation: f=1/(2π√LC). This allows the LC parallel resonance circuit 7 to have a high impedance with respect to the control pulse.
The regulator IC 8 generates a predetermined level of power supply voltage by using the direct current for power source supplied via the LC parallel resonance circuit 7 to carry out the supply of electricity to a signal amplifying and frequency converting circuit 9 and a pulse detecting circuit 10 which are included in the high impedance circuit 5.
The pulse detecting circuit 10 is caused by the power supplied from the regulator IC 8 to output a satellite selecting instruction to select a target satellite for receiving signals to the signal amplifying and frequency converting circuit 9, in response to the control pulse transmitted from the receiver. The signal amplifying and frequency converting circuit 9 is caused by the power supplied from the regulator IC 8 to switch between the receiving horn 2 and the receiving horn 3 in response to the satellite selecting instruction sent from the pulse detecting circuit 10. Then, the signal amplifying and frequency converting circuit 9 receives and amplifies signals of the BS broadcast or the CS broadcast, further converts the signals into IF signals, and outputs the IF signals from a coupling capacitor c3 to the receiver via the terminal 4.
Here, the signal input level of the control pulse supplied to the pulse detecting circuit 10 is determined by a relative ratio of an output impedance of the receiver as a signal source, an impedance of lines such as a cable, an input impedance of the pulse detecting circuit 10, to the LC parallel resonance circuit 7. However, a small level of signal input causes the pulse detecting circuit 10 to misread signals. It is therefore important that the LNB 1, which is set at the places such as a rooftop of a house using a long cable, has the input impedance much larger than the impedance of the cable to keep a large level of the signal input to the LNB 1. Further, the LNB 1, which is set at the places such as a rooftop, may be hit by a stroke of lightning, so that it is necessary for the LNB 1 to withstand the lightning stroke.
Therefore, the high impedance circuit 5 is caused to have a high input impedance. Further, the direct current for power source is supplied to the high impedance circuit 5. At an input stage of the low impedance circuit 6 which is caused to have a low input impedance by the input pass capacitor c1, provided is the LC parallel resonance circuit 7 for raising circuit impedance. Adjustment of the resonance frequency as described previously causes the LC parallel resonance circuit 7 to occur resonance when the control pulse is inputted, so that a high impedance is developed in the LC parallel resonance circuit 7. At this point, the circuits seen from an input side are the high impedance circuit 5 and the LC parallel resonance circuit 7. That is, the high impedance causes the control pulse to be inputted to the pulse detecting circuit 10 at the sufficient level of the signal input.
In the LNB 1 which is arranged as described above, a surge absorber zd is provided to withstand surge voltage caused by sources such as a lightning. However, there is a problem that the addition of the surge voltage causes counter electromotive force which is many times larger than the surge voltage across the inductor 1 polarized as shown in FIG. 4, resulting in the destruction of peripheral circuits such as the high impedance circuit 5.